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1. Field of the Invention
The present invention generally relates to a system and method for deploying and installing cable on the ocean floor with a remotely operated vehicle (ROV). More particularly, the present invention relates to a system to use an ROV to deploy and bury cables along the ocean bottom utilizing a series of cables pre-wound on a set of reels.
2. Background of the Invention
Dating back to the first transatlantic telecommunications cables, there has often been a need to lay cable of various configurations for a wide array or purposes in sub-sea environments. Traditional cable laying operations are performed by spooling the cable off of a large reel on the back of a ship as the body of water is traversed. Traditionally, sensor, or data collection, cable is similarly deployed. Although effective for deploying cabling, such ship laid operations leave a lot to be desired when any amount of precision is required in the placement of the laid cable. Whereas transatlantic communications cables are able to perform their functions properly as long as there is a link between the starting and ending points, sensor cable is often required to be in a specific position in order to measure its intended information. One such measurement system that requires relatively complex patterns and precise placement is cabling for the purpose of collecting seismic surveys.
Seismic surveys are conducted for the exploration of hydrocarbon producing zones and reservoirs. One method includes the placing of an array of seismic receivers upon the surface of the earth. When in place, seismic sources are triggered to generate seismic waves that travel downward through the earth and are reflected off of underground deposits or changes in formation. The reflected seismic waves then return upward and are recorded by the seismic receivers at the surface. Data from the generation of the seismic waves at each source to the reception of the seismic waves at each receiver is recorded and is entered into a computer to give the operator an indication of the depth and composition of the formation and any mineral deposits encapsulated therein.
Typical seismic surveys performed today are capable of producing three-dimensional (3-D) surveys of the earth""s outer crust. The surveys are generated by placing an array of seismic sensors in the ground prior to drilling, acquiring seismic measurements, and retrieving the array following data acquisition. Drillers then use the data collected by the 3-D seismic array to help find petroleum reservoir deposits and to aid them in making decisions on potential well locations and configurations.
To maximize the production of hydrocarbons from an underground reservoir or formation, it is important to determine the development and behavior of the reservoir during the production life of the reservoir and to foresee changes which will affect the reservoir. More recently, four-dimensional (4-D) seismic survey systems have been used to produce 3-D measurements as before, but over extended periods of time. Such an a arrangement allows production managers to monitor the long term effects of drilling and producing petroleum products from the formation underneath. For example, a production field with several producing wells can be monitored with repeat measurements over time to determine if one well in the field is having an adverse affect on the productivity of another well in the same field. Furthermore, a 4-D seismic array can also keep operators informed as to the amount of petroleum remaining within the reservoir and possible courses of action to maximize its production. Four-dimensional seismic systems allow operators to monitor the long term performance and productivity of their valuable petroleum assets. By obtaining a series of records over time, it is possible to monitor the movement of fluid in and out of the reservoirs, and to thereby obtain reservoir information needed to improve the amount of and the efficiency with which the hydrocarbons are produced.
For long-term recording, it is desirable that the emplaced sensors be substantially stationary throughout their life. Movement in long-term sensors can distort the accuracy of data collected over long periods of time. Any change in position of the sensors may cause inconsistency in the data collected from one time period to another. For example, a production company that desires to monitor a particular reservoir for a period of 20 years needs to ensure that the array of seismic cables has had only negligible positional changes over those 20 years. Furthermore, to maximize seismic array sensitivity, the sensors must be properly coupled to the ocean floor from which they are to measure seismic activity. To prevent sensor movement and to facilitate that proper coupling, seismic cable with attached sensors are preferably buried in the ocean floor thereby embedding the sensors and causing the ocean floor to maintain the sensors"" position. For land based arrays, this process is relatively simple, using heavy machinery to dig trenches to bury the sensor array. For subsea seismic, the process is more complicated.
Sub-sea seismic cables are typically deployed off the back of a slowly moving ship. The cable, preferably constructed as a reinforced cable, is loaded upon the deck of the ship in large spools. The seismic sensors are attached to the cable and are of greater diameter than the cable. Therefore, it is important that care be taken while loading and unloading the cable on the large spools. Furthermore, spools must include a large enough inner diameter so as to prevent damage to the sensors when the cable is wrapped thereupon. Once the cable is loaded upon the spools and is on board the ship, the cable can be paid out from the deck of the ship to the ocean floor below. In deep water, the weight of the cable extending from the vessel to the ocean floor together with the movement of the vessel creates substantial tension and stress on the cable. Because the construction of the cabling is relatively delicate, great care must be taken not to over stress the cable as it is laid as the seismic sensor cable experiences its greatest threat of damage during deployment.
Once laid on the floor of the ocean, in order to maximize performance, it is preferred that the sensor cables be buried in a predetermined array on the ocean floor. To accomplish this task, remotely operated vehicles (ROV""s) are specially equipped with a jetting package to bury the seismic cable. A jetting package typically includes jet nozzles and a depression arm. The jetting package is designed to be carried underneath an ROV and follow along the path of the laid cable. As the ROV pilot flies the ROV into the ocean floor, following the laid cable, the jet nozzles inject pressurized water into the ocean bottom and, depending on soil composition, either liquefy or create a temporary trench in the ocean floor. As nozzles create the trench or liquefied region, the depression arm pushes the cable into the trench with the loose silt and ocean floor material filling in behind, leaving the cable in a buried state. An example of a jetting package of this type used to bury already-laid cable on the ocean floor in this manner is manufactured by Perry Tritech. Once buried, the seismic cable is now suited to perform seismic readings throughout the life of the field. Because of the time, expense and stresses to the cable, seismic cable is preferably not retrieved and reused following production.
The primary drawback to seismic array systems currently in use for sub-sea environments is their high cost of installation, their low flexibility of placement, and the poor reliability of their sub-sea connections. Because of the manner in which they are laid from a ship, the network of seismic sensors is often constructed as a series of separate cables. A plurality of electromechanical connections are made up on the ocean floor to create the network. Because of the nature of electromechanical connectors in marine and high stress environments, the connections are often characterized by low reliability. It would be preferable for a system to deploy a network of seismic cabling to be developed to either eliminate or reduce the need for electromechanical connectors and to dramatically reduce the stress experienced by the cable while it is being laid. Furthermore, current ship laying cable operations are limited in the pattern of the array that can be laid on the ocean floor. A ship operating in several hundred meters of water cannot lay cable on the ocean floor so as to cause the cable to have precisely angled turns without the use of a piling on the ocean floor around which to direct the cable. Thus prior art systems cannot easily produce precise patterns or complex arrays of cable at the ocean floor, instead being limited to long substantially straight and large radius curved sections. A system to lay a more robust network of seismic cables with greater precision and reduced potential for cable damage would be highly desirable to oilfield exploration and operation companies. The present invention overcomes the deficiencies of the prior art.
The underwater cable deployment system of the present invention includes a remotely operated vehicle (ROV) for deploying a series of cables, pre-wound on a set of reels, on the ocean floor in a preferred sensor array at a desired field of investigation. Preferably all cable connections are made up prior to deployment and placed upon a pallet that is delivered to the desired field of investigation. The pallet preferably includes all equipment (distribution hubs, communication riser, etc.) that are needed to communicate with the sensor array and is delivered to the ocean floor by a crane or other lowering device with the individual sensor array cables on reels to be deployed later by the ROV. The ROV includes a reel deployer configured to pay out and apply back tension to the sensor cable. Optionally, the ROV can include a jetting package configured to simultaneously bury the sensor cable while the cable is paid out. With the sensors deployed and buried, the ROV returns to the surface with a communications riser cable so that a surface facility can interface with and receive data transmissions from the sensor array.
The preferred embodiments of the present invention provide a system to easily and precisely deploy a sub-sea sensor array into various complex sensor patterns using a remotely operated vehicle. By avoiding suspending the cable from the vessel to the ocean floor, the stress experienced by the sensor cable is minimized. Furthermore, the increased precision of the deployment system allows the sensor cables to be laid and buried in predetermined configurations on the sea floor which are not possible with current systems. These and other advantages of the present invention will become apparent on reading the detailed description of the invention in conjunction with the drawings.